Compression braking, or engine braking, may be employed to assist and supplement wheel brakes in slowing heavy machines, such as, on-highway trucks, construction machines, earthmoving machines, and the like. Compression braking may convert an internal combustion engine from a power generating unit into a power consuming air compressor. A compression braking system may actuate a cylinder exhaust valve and compressed air from the compression stroke of the engine is released through the cylinder exhaust valve when the piston in the cylinder nears the top-dead-center position. Generally, the exhaust valve is actuated by a rocker arm. The rocker arm is often operatively engaged with the exhaust valve by way of a valve bridge. The rocking motion of the rocker arm presses down on the valve bridge which in turn opens the exhaust valve, releasing the compressed air.
A hydraulic lash adjuster is often disposed on the rocker arm and may be directly on or above the exhaust valve so as to maintain zero clearance between the exhaust valve and its corresponding valve seat. The hydraulic lash adjuster may include a hollow, sliding plunger operated by a hydraulic fluid, such as engine oil. When the exhaust valve is closed, the hydraulic lash adjuster may be free to fill with the hydraulic fluid. When the exhaust valve is opened, the fluid supply to the hydraulic lash adjuster may be blocked and fluid pressure within the hydraulic lash adjuster prevents the plunger from sliding.
At the end of a compression braking operation, a load associated with the rocking motion of the rocker arm is removed from the hydraulic lash adjuster. Because a load is absent from the plunger, the hydraulic lash adjuster may pump-up, wherein the plunger may excessively protrude from the hydraulic lash adjuster, resulting in the exhaust valve not fully seating. The partial opening of the exhaust valve may ultimately result in poor emissions and valve guttering.
One method of improving the performance of a compression braking system under such conditions is described in U.S. Pat. No. 6,718,940 B2 (the '940 patent) to Usko, issued on Apr. 13, 2004. The '940 patent describes a system for use in a diesel engine having a compression release retarder and is directed to an improved assembly for operating a valve in an engine cylinder. The assembly includes a rocker shaft, a rocker arm pivotally mounted on the rocker shaft for operating the valve, the rocker arm including a cavity at a valve actuation end, a hydraulic lash adjuster disposed within the rocker arm cavity for adjusting a lash between the rocker arm and the valve. The assembly further includes a fluid supply assembly for supplying fluid to the hydraulic lash adjuster to operate the hydraulic lash adjuster. The fluid supply assembly may include fluid passages in the rocker arm and a solenoid valve on the rocker shaft to supply hydraulic fluid to the hydraulic lash adjuster during a positive power mode of engine operation, and to cut off hydraulic fluid to the hydraulic lash adjuster during an engine braking mode.
The compression braking system of the present disclosure is directed towards improvements to the existing technology.